Computer Science
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..739g&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Computer Science
Scientific paper
Abstract Venus has a negligible intrinsic magnetic moment with an upper limit a factor 10-5 of earth's [1]. This entails that the ionosphere is vulnerable to scavenging by the solar wind. However, magnetic fields may be induced in the ionosphere by interaction with the interplanetary magnetic field frozen-in to the solar wind. The presence of small scale magnetic structures in the dayside ionosphere of the planet Venus has been long established and were first observed in Pioneer Venus Orbiter (PVO) data in 1979 [2] during the run up to solar maximum. These ionospheric `flux ropes' were observed in over 70% of passes in which the orbit of PVO intersected the dayside ionosphere [3]. Magnetic flux ropes are identified as brief, discrete disturbances from any background magnetic field, lasting a few seconds with a magnitude of up to many 10's of nano-Teslas in strength [3, 4]. Flux ropes have a strong central, axial field, that is wrapped with field lines of weakening strength and increased helical angle with distance from the central field lines [4]. Due to this particular structure, flux ropes present a specific signature in the three variance projections (also known as a hodogram) when minimum variance analysis is applied to the magnetic data set [2]. With Venus Express now in operational orbit around the planet, flux ropes are being observed in the data retrieved by the magnetometers (MAG [5]) onboard. The magnetic data used in this analysis is the 1Hz data set provided by H. Wei (of UCLA). Variance projections have been produced for several structures in 2006, revealing them to be flux ropes (see figure 1). Using the Ion Mass Analyser (IMA; part of the ASPERA-4 package [6]) and MAG, the ion composition within the ropes and the effect of such magnetic structures upon ionospheric erosion is being studied. Where flux ropes have been evident in the magnetic data, ion spectra have been produced in an attempt to deduce any compositional differences between a flux rope free ionosphere and one in which flux ropes have been detected. The spectra have been constructed to show protons, alpha particles, singly ionised helium and singly ionised oxygen, displaying a mix of solar wind and atmospheric particles. ELS (electron spectrometer [6]) data sets have been inspected to ascertain if the ropes occur deep in the ionosphere or at its fringes. The case that is primarily being studied occurs on 2006-07-11, DOY 192. The spacecraft travels along a near-terminator orbit from midnight to midday. H+ and He++ spectra show double energy populations in the region of the flux rope. These double populations show strongly in the count rate. A double population can also be weakly observed in the O+ spectra. The energies of these populations are approximately 1KeV and 20eV, indicating a mix of solar wind and planetary ions. The electron data does not appear to show any signature of the ionosphere, but is suspected to be in a position at the fringe of the night-side ionopause (see figure 2). In order to determine the pressure balance across the ropes the average plasma and magnetic pressures both across the ropes and during a three minute period either side of the ropes has been calculated. Flux ropes with a force-free nature have increased magnetic pressure but do not have a decreased thermal pressure. Indicating that particle density within the flux rope will not be reduced [7]. For ions to be eroded from the atmosphere by flux ropes, the ropes would need to be force-free, i.e., non-density depleted.
Grande Marco
Guymer G.
Whittaker Ian
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